Power conversion device and power conversion method

ABSTRACT

A power conversion method of a power conversion device including a primary side port disposed in a primary side circuit and a secondary side port disposed in a secondary side circuit magnetically coupled to the primary side circuit with a transformer, the power conversion device adjusting transmission power transmitted between the primary side circuit and the secondary side circuit by changing a phase difference between switching of the primary side circuit and switching of the secondary side circuit, and changing a voltage of the secondary side port by a DC-DC converter connected to the secondary side port, the power conversion method including: monitoring a voltage ratio of a voltage of the primary side port and the voltage of the secondary side port; and causing the DC-DC converter to operate when the voltage ratio deviates from the reference value by the specified value or more.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-081405 filed onApr. 10, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power conversion device and a powerconversion method.

2. Description of Related Art

A power conversion device is known which adjusts transmission powertransmitted between a primary side conversion circuit including aprimary side port and a secondary side conversion circuit including asecondary side port and being magnetically coupled to the primary sideconversion circuit with a transformer depending on a phase difference φ(for example, see Japanese Patent Application Publication No.2011-193713 (JP 2011-193713 A)).

In the power conversion device as described above, it is assumed that avoltage ratio of a voltage of the primary side port and a voltage of thesecondary side port is constant (reference value).

However, for example, when the secondary side port is connected to ahigh voltage battery of a hybrid vehicle, the voltage of the secondaryside port varies significantly (±50V or more). Since the voltage ratiodeviates from the reference value, there is a possibility that thetransmission power decreases.

SUMMARY OF THE INVENTION

Therefore, an aspect of the invention provides for suppressing adecrease in transmission power.

According to an aspect of the invention, there is provided a powerconversion method of a power conversion device including a primary sideport disposed in a primary side circuit and a secondary side portdisposed in a secondary side circuit magnetically coupled to the primaryside circuit with a transformer, the power conversion device adjustingtransmission power transmitted between the primary side circuit and thesecondary side circuit by changing a phase difference between switchingof the primary side circuit and switching of the secondary side circuit,and changing a voltage of the secondary side port by a DC-DC converterthat is connected to the secondary side port, the power conversionmethod including: monitoring a voltage ratio of a voltage of the primaryside port and the voltage of the secondary side port; determiningwhether the voltage ratio deviates from a reference value by a specifiedvalue or more; and causing the DC-DC converter to operate when thevoltage ratio deviates from the reference value by the specified valueor more.

According to the aspect of the invention, it is possible to suppress thedecrease in the transmission power.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a block diagram illustrating a configuration example of apower supply device as an embodiment of a power conversion device;

FIG. 2 is a block diagram illustrating a configuration example of acontrol unit;

FIG. 3 is a timing diagram illustrating a switching example of a primaryside circuit and a secondary side circuit;

FIG. 4 is a block diagram illustrating a configuration example of acontrol unit;

FIG. 5A is a graph illustrating a relationship between transmissionpower P and a voltage ratio M;

FIG. 5B is a graph illustrating the relationship between thetransmission power P and the voltage ratio M;

FIG. 6A is a graph illustrating the relationship between thetransmission power P and the voltage ratio M;

FIG. 6B is a graph illustrating the relationship between thetransmission power P and the voltage ratio M; and

FIG. 7 is a flowchart illustrating an example of a power conversionmethod.

DETAILED DESCRIPTION OF EMBODIMENTS

<Configuration of Power Supply Device 101>

FIG. 1 is a block diagram illustrating a configuration example of apower supply device 101 as an embodiment of a power conversion device.The power supply device 101 is, for example, a power supply systemincluding a power supply circuit 10, a control unit 50, a sensor unit70, and a step-up/down DC-DC converter 80. The power supply device 101is a system that is mounted on a vehicle such as an automobile and thatdistributes power to in-vehicle loads. Specific examples of the vehicleinclude a hybrid vehicle, a plug-in hybrid vehicle, and an electricautomobile.

For example, the power supply device 101 includes a first input/outputport 60 a connected to a primary side high voltage system load (forexample, an electric power steering device (EPS)) 61 a and a secondinput/output port 60 c connected to a primary side low voltage systemload (for example, an electronic control unit (ECU) and an electroniccontrol brake system (ECB)) 61 c and a primary side low voltage systempower supply (for example, an auxiliary battery) 62 c as primary sideports. The primary side low voltage system power supply 62 c suppliespower to the primary side low voltage system load 61 c operating in thesame voltage system (for example, 12 V system) as the primary side lowvoltage system power supply 62 c. Further, the primary side low voltagesystem power supply 62 c supplies power, which has been stepped up by aprimary side conversion circuit 20 disposed in the power supply circuit10, to the primary side high voltage system load 61 a operating in avoltage system (for example, 48 V system higher than the 12 V system)different from the primary side low voltage system power supply 62 c. Aspecific example of the primary side low voltage system power supply 62c is a secondary battery such as a lead battery.

The power supply device 101 includes a third input/output port 60 bconnected to the step-up/down DC-DC converter 80, a secondary side highvoltage system load 61 b and a secondary side high voltage system powersupply (for example, a main battery) 62 b and a fourth input/output port60 d connected to a secondary side low voltage system load 61 d assecondary side ports. The secondary side high voltage system powersupply 62 b supplies power to the secondary side high voltage systemload 61 b operating in the same voltage system (for example, 288 Vsystem higher than the 12 V system and the 48 V system) as the secondaryside high voltage system power supply 62 b. The secondary side highvoltage system power supply 62 b supplies power, which has been steppeddown by a secondary side conversion circuit 30 disposed in the powersupply circuit 10, to the secondary side low voltage system load 61 doperating in a voltage system (for example, 72 V system lower than the288 V system) different from the secondary side high voltage systempower supply 62 b. A specific example of the secondary side high voltagesystem power supply 62 b is a secondary battery such as a lithium ionbattery.

The power supply circuit 10 is a power conversion circuit that includesthe aforementioned four input/output ports and that has a function ofselecting two input/output ports out of the four input/output ports andperforming power conversion between the selected two input/output ports.The power supply device 101 including the power supply circuit 10 may bea device that includes three or more input/output ports and that canconvert power between two input/output ports out of the three or moreinput/output ports. For example, the power supply circuit 10 may be, forexample, a circuit that includes three input/output ports other than thefourth input/output port 60 d.

Port power Pa, Pc, Pb, Pd are input/output power (input power or outputpower) at the first input/output port 60 a, the second input/output port60 c, the third input/output port 60 b, and the fourth input/output port60 d. Port voltages Va, Vc, Vb, Vd are input/output voltages (an inputvoltage or an output voltage) at the first input/output port 60 a, thesecond input/output port 60 c, the third input/output port 60 b, and thefourth input/output port 60 d. Port currents Ia, Ic, Ib, Id areinput/output currents (an input current or an output current) at thefirst input/output port 60 a, the second input/output port 60 c, thethird input/output port 60 b, and the fourth input/output port 60 d.

The power supply circuit 10 includes a capacitor C1 disposed at thefirst input/output port 60 a, a capacitor C3 disposed at the secondinput/output port 60 c, a capacitor C2 disposed at the thirdinput/output port 60 b, and a capacitor C4 disposed at the fourthinput/output port 60 d. Specific examples of the capacitors C1, C2, C3,C4 include a film capacitor, an aluminum electrolytic capacitor, aceramic capacitor, and a solid polymer capacitor.

The capacitor C1 is inserted between a high potential terminal 613 ofthe first input/output port 60 a and a low potential terminal 614 of thefirst input/output port 60 a and the second input/output port 60 c. Thecapacitor C3 is inserted between a high potential terminal 616 of thesecond input/output port 60 c and the low potential terminal 614 of thefirst input/output port 60 a and the second input/output port 60 c. Thecapacitor C2 is inserted between a high potential terminal 618 of thethird input/output port 60 b and a low potential terminal 620 of thethird input/output port 60 b and the fourth input/output port 60 d. Thecapacitor C4 is inserted between a high potential terminal 622 of thefourth input/output port 60 d and the low potential terminal 620 of thethird input/output port 60 b and the fourth input/output port 60 d.

The capacitors C1, C2, C3, C4 may be disposed inside the power supplycircuit 10 or may be disposed outside the power supply circuit 10.

The power supply circuit 10 is a power conversion circuit including theprimary side conversion circuit 20 and the secondary side conversioncircuit 30. The primary side conversion circuit 20 and the secondaryside conversion circuit 30 are connected to each other via a primaryside magnetic coupling reactor 204 and a secondary side magneticcoupling reactor 304 and are magnetically coupled with a transformer 400(center-tap transformer). The primary side ports including the firstinput/output port 60 a and the second input/output port 60 c and thesecondary side ports including the third input/output port 60 b and thefourth input/output port 60 d are connected to each other via thetransformer 400.

The primary side conversion circuit 20 is a primary side circuitincluding a primary side full bridge circuit 200, the first input/outputport 60 a, and the second input/output port 60 c. The primary side fullbridge circuit 200 is a primary side power conversion unit including aprimary side coil 202 of the transformer 400, the primary side magneticcoupling reactor 204, a primary side first upper arm U1, a primary sidefirst lower arm /U1, a primary side second upper arm V1, and a primaryside second lower arm /V1. Here, the primary side first upper arm U1,the primary side first lower arm /U1, the primary side second upper armV1, and the primary side second lower arm /V1 are, for example,switching elements including an N-channel MOSFET and a body diode as aparasitic element of the MOSFET. A diode may be additionally connectedin parallel to the MOSFET.

The primary side full bridge circuit 200 includes a primary sidepositive electrode bus line 298 connected to the high potential terminal613 of the first input/output ports 60 a and a primary side negativeelectrode bus line 299 connected to the low potential terminal 614 ofthe first input/output port 60 a and the second input/output port 60 c.

A primary side first arm circuit 207 in which the primary side firstupper arm U1 and the primary side first lower arm /U1 are connected inseries is disposed between the primary side positive electrode bus line298 and the primary side negative electrode bus line 299. The primaryside first arm circuit 207 is a primary side first power conversioncircuit unit (primary side U-phase power conversion circuit unit) thatcan perform a power conversion operation by ON/OFF switching operationsof the primary side first upper arm U1 and the primary side first lowerarm /U1. A primary side second arm circuit 211 in which the primary sidesecond upper arm V1 and the primary side second lower arm /V1 areconnected in series is disposed in parallel to the primary side firstarm circuit 207 between the primary side positive electrode bus line 298and the primary side negative electrode bus line 299. The primary sidesecond arm circuit 211 is a primary side second power conversion circuitunit (primary side V-phase power conversion circuit unit) that canperform a power conversion operation by ON/OFF switching operations ofthe primary side second upper arm V1 and the primary side second lowerarm /V1.

A bridge part connecting a midpoint 207 m of the primary side first armcircuit 207 and a midpoint 211 m of the primary side second arm circuit211 is provided with the primary side coil 202 and the primary sidemagnetic coupling reactor 204. The connection relationship of the bridgepart will be described below in more detail. The midpoint 207 m of theprimary side first arm circuit 207 is connected to one end of a primaryside first reactor 204 a of the primary side magnetic coupling reactor204. The other end of the primary side first reactor 204 a is connectedto one end of the primary side coil 202. The other end of the primaryside coil 202 is connected to one end of a primary side second reactor204 b of the primary side magnetic coupling reactor 204. The other endof the primary side second reactor 204 b is connected to the midpoint211 m of the primary side second arm circuit 211. The primary sidemagnetic coupling reactor 204 includes the primary side first reactor204 a and the primary side second reactor 204 b magnetically coupled tothe primary side first reactor 204 a with a coupling coefficient k1.

The midpoint 207 m is a primary side first intermediate node between theprimary side first upper arm U1 and the primary side first lower arm/U1, and the midpoint 211 m is a primary side second intermediate nodebetween the primary side second upper arm V1 and the primary side secondlower arm /V1.

The first input/output port 60 a is a port disposed between the primaryside positive electrode bus line 298 and the primary side negativeelectrode bus line 299. The first input/output port 60 a includes theterminal 613 and the terminal 614. The second input/output port 60 c isa port disposed between the primary side negative electrode bus line 299and the center tap 202 m of the primary side coil 202. The secondinput/output port 60 c includes the terminal 614 and the terminal 616.

The port voltage Va of the first input/output port 60 a and the portvoltage Vc of the second input/output port 60 c vary depending on thevoltage of the primary side low voltage system power supply 62 c.

The center tap 202 m is connected to the high potential terminal 616 ofthe second input/output port 60 c. The center tap 202 m is anintermediate connecting point between a primary side first winding 202 aand a primary side second winding 202 b disposed in the primary sidecoil 202.

The secondary side conversion circuit 30 is a secondary side circuitincluding a secondary side full bridge circuit 300, the thirdinput/output port 60 b, and the fourth input/output port 60 d. Thesecondary side full bridge circuit 300 is a secondary side powerconversion unit including a secondary side coil 302 of the transformer400, the secondary side magnetic coupling reactor 304, a secondary sidefirst upper arm U2, a secondary side first lower arm /U2, a secondaryside second upper arm V2, and a secondary side second lower arm /V2.Here, the secondary side first upper arm U2, the secondary side firstlower arm /U2, the secondary side second upper arm V2, and the secondaryside second lower arm /V2 are, for example, switching elements includingan N-channel MOSFET and a body diode as a parasitic element of theMOSFET. A diode may be additionally connected in parallel to the MOSFET.

The secondary side full bridge circuit 300 includes a secondary sidepositive electrode bus line 398 connected to the high potential terminal618 of the third input/output ports 60 b and a secondary side negativeelectrode bus line 399 connected to the low potential terminal 620 ofthe third input/output port 60 b and the fourth input/output port 60 d.

A secondary side first arm circuit 307 in which the secondary side firstupper arm U2 and the secondary side first lower arm /U2 are connected inseries is disposed between the secondary side positive electrode busline 398 and the secondary side negative electrode bus line 399. Thesecondary side first arm circuit 307 is a secondary side first powerconversion circuit unit (secondary side U-phase power conversion circuitunit) that can perform a power conversion operation by ON/OFF switchingoperations of the secondary side first upper arm U2 and the secondaryside first lower arm /U2. A secondary side second arm circuit 311 inwhich the secondary side second upper arm V2 and the secondary sidesecond lower arm /V2 are connected in series is disposed in parallel tothe secondary side first arm circuit 307 between the secondary sidepositive electrode bus line 398 and the secondary side negativeelectrode bus line 399. The secondary side second arm circuit 311 is asecondary side second power conversion circuit unit (secondary sideV-phase power conversion circuit unit) that can perform a powerconversion operation by ON/OFF switching operations of the secondaryside second upper arm V2 and the secondary side second lower arm /V2.

A bridge part connecting a midpoint 307 m of the secondary side firstarm circuit 307 and a midpoint 311 m of the secondary side second armcircuit 311 is provided with the secondary side coil 302 and thesecondary side magnetic coupling reactor 304.

The connection relationship of the bridge part will be described belowin more detail. The midpoint 307 m of the secondary side first armcircuit 307 is connected to one end of a secondary side first reactor304 a of the secondary side magnetic coupling reactor 304. The other endof the secondary side first reactor 304 a is connected to one end of thesecondary side coil 302. The other end of the secondary side coil 302 isconnected to one end of a secondary side second reactor 304 b of thesecondary side magnetic coupling reactor 304. The other end of thesecondary side second reactor 304 b is connected to the midpoint 311 mof the secondary side second arm circuit 311. The secondary sidemagnetic coupling reactor 304 includes the secondary side first reactor304 a and the secondary side second reactor 304 b magnetically coupledto the secondary side first reactor 304 a with a coupling coefficientk2.

The midpoint 307 m is a secondary side first intermediate node betweenthe secondary side first upper arm U2 and the secondary side first lowerarm /U2, and the midpoint 311 m is a secondary side second intermediatenode between the secondary side second upper arm V2 and the secondaryside second lower arm /V2.

The third input/output port 60 b is a port disposed between thesecondary side positive electrode bus line 398 and the secondary sidenegative electrode bus line 399. The third input/output port 60 bincludes the terminal 618 and the terminal 620. The fourth input/outputport 60 d is a port disposed between the secondary side negativeelectrode bus line 399 and the center tap 302 m of the secondary sidecoil 302. The fourth input/output port 60 d includes the terminal 620and the terminal 622.

The port voltage Vb of the third input/output port 60 a and the portvoltage Vd of the fourth input/output port 60 d vary depending on thevoltage of the secondary side low voltage system power supply 62 b.

The center tap 302 m is connected to the high potential terminal 622 ofthe fourth input/output port 60 d. The center tap 302 m is anintermediate connecting point between a secondary side first winding 302a and a secondary side second winding 302 b disposed in the secondaryside coil 302.

In FIG. 1, the power supply device 101 includes a sensor unit 70. Thesensor unit 70 is a detection unit that detects an input/output value Yat at least one of the first to fourth input/output ports 60 a, 60 c, 60b, 60 d with a predetermined detection cycle and that outputs a detectedvalue Yd corresponding to the detected input/output value Y to thecontrol unit 50. The detected value Yd may be a detected voltageobtained by detecting an input/output voltage, a detected currentobtained by detecting an input/output current, or may be detected powerobtained by detecting input/output power. The sensor unit 70 may bedisposed inside the power supply circuit 10 or may be disposed outsidethe power supply circuit 10.

The sensor unit 70 includes, for example, a voltage detecting unit thatdetects an input/output voltage generated in at least one port of thefirst to fourth input/output ports 60 a, 60 c, 60 b, 60 d. The sensorunit 70 includes, for example, a primary side voltage detecting unitthat outputs the detected voltage of at least one of the input outputvoltage Va and the input/output voltage Vc as a primary side detectedvoltage value and a secondary side voltage detecting unit that outputsthe detected voltage of at least one of the input/output voltage Vb andthe input/output voltage Vd as a secondary side detected voltage value.

The voltage detecting unit of the sensor unit 70 includes, for example,a voltage sensor that monitors the input/output voltage value of atleast one port and a voltage detection circuit that outputs a detectedvoltage corresponding to the input/output voltage value monitored by thevoltage sensor to the control unit 50.

The sensor unit 70 includes, for example, a current detecting unit thatdetects an input/output current flowing in at least one port of thefirst to fourth input/output ports 60 a, 60 c, 60 b, 60 d. The sensorunit 70 includes a primary side current detecting unit that outputs thedetected current of at least one of the input/output current Ia and theinput/output current Ic as a primary side detected current value and asecondary side current detecting unit that outputs the detected currentof at least one of the input/output current Ib and the input/outputcurrent Id as a secondary side detected current value.

The current detecting unit of the sensor unit 70 includes, for example,a current sensor that monitors the input/output current value of atleast one port and a current detection circuit that outputs a detectedcurrent corresponding to the input/output current value monitored by thecurrent sensor to the control unit 50.

In FIG. 1, the power supply device 101 includes the step-up/down DC-DCconverter 80.

The step-up/down DC-DC converter 80 operates when a voltage ratio Mbetween the input/output voltage of the first input/output port 60 a(port voltage Va) and the input/output voltage of the third input/outputport 60 b (port voltage Vb) deviates from a reference value by aspecified value or more. Further, the step-up/down DC-DC converter 80does not operate when the voltage ratio M does not deviate from thereference value by the specified value or more. That is, thestep-up/down DC-DC converter 80 operates or does not operate based onthe voltage ratio M.

Note that, the reference value represents a voltage ratio (=N) when theport voltage Va and the port voltage Vb are balanced (port voltageVa:port voltage Vb=1: N (N is a turns ratio of the transformer 400)) anddesired power is transmitted between the primary side conversion circuit20 and the secondary side conversion circuit 30. Further, the specifiedvalue represents a limit value that allows the voltage ratio M todeviate from the reference value (=N) while the power that istransmitted between the primary side conversion circuit 20 and thesecondary side conversion circuit 30 does not decrease.

For example, when the voltage ratio M satisfies the reference value±thespecified value ((the reference value−the specified value)<the voltageratio M<(the reference value+the specified value)), since the portvoltage Vb holds a value within a specified range, the transmissionpower does not decrease. For example, when the voltage ratio M deviatesby the specified value or more (the voltage ratio M≦(the referencevalue−the specified value), or (the reference value+the specifiedvalue)≦the voltage ratio M), since the port voltage Vb does not hold avalue within the specified range, the transmission power decreases.

The power supply device 101 includes the control unit 50. The controlunit 50 is, for example, an electronic circuit including a microcomputer having a CPU built therein. The control unit 50 may be disposedinside the power supply circuit 10 or may be disposed outside the powersupply circuit 10.

The control unit 50 controls the power conversion operation performed bythe power supply circuit 10 in a feedback manner by changing the valueof a predetermined control parameter X, and can adjust the input/outputvalues Y at the first to fourth input/output ports 60 a, 60 c, 60 b, 60d of the power supply circuit 10. Examples of the main control parameterX include two types of control parameters of a phase difference φ and aduty ratio D (on-time δ).

The phase difference φ is a difference in switching timing (time lag)between the power conversion circuit units of the same phase in theprimary side full bridge circuit 200 and the secondary side full bridgecircuit 300. The duty ratio (on-time δ) is a duty ratio (on-time) of aswitching waveform in the power conversion circuit units in the primaryside full bridge circuit 200 and the secondary side full bridge circuit300.

These two control parameters X can be controlled independently of eachother. The control unit 50 changes the input/output values Y at theinput/output ports of the power supply circuit 10 by duty ratio controland/or phase control of the primary side full bridge circuit 200 and thesecondary side full bridge circuit 300 using the phase difference φ andthe duty ratio D (on-time δ).

The control unit 50 controls the power conversion operation of the powersupply circuit 10 in a feedback manner so that the detected value Yd ofthe input/output value Y in at least one port of the first to fourthinput/output ports 60 a, 60 c, 60 b, 60 d converges on a target value Yoset at the port. The target value Yo is a command value set by thecontrol unit 50 or a predetermined device other than the control unit50, for example, on the basis of drive conditions defined for each load(for example, the primary side low voltage system load 61 c) connectedto the respective input/output ports. The target value Yo serves as anoutput target value when electric power is output from the port, servesas an input target value when electric power is input to the port, andmay be a target voltage value, may be a target current value, or may bea target power value.

Further, the control unit 50 controls the power conversion operation ofthe power supply circuit 10 in a feedback manner to change the phasedifference φ so that transmission power P transmitted via thetransformer 400 between the primary side conversion circuit 20 and thesecondary side conversion circuit 30 converges on preset targettransmission power. The transmission power is also referred to as anamount of power transmitted. The target transmission power is a commandvalue set by the control unit 50 or a predetermined device other thanthe control unit 50, for example, on the basis of the difference betweenthe detected value Yd and the target value Yo at a certain port.

The control unit 50 detects the port voltage Va and the port voltage Vb,monitors the voltage ratio M (ratio of the port voltage Va and the portvoltage Vb), causes the step-up/down DC-DC converter 80 to operate ornot to operate, and controls the transmission power transmitted betweenthe primary side conversion circuit 20 and the secondary side conversioncircuit 30.

For example, when the voltage ratio M deviates from the reference valueby the specified value or more, the control unit 50 causes thestep-up/down DC-DC converter 80 to operate. Moreover, the control unit50 changes the port voltage Vb to make it hold a value within thespecified range.

Thus, the power supply device 101 can keep the voltage ratio Msubstantially constant (a value in an allowable range: (the referencevalue−the specified value)<the voltage ratio M<(the reference value+thespecified value)), to suppress a decrease in the power transmittedbetween the primary side conversion circuit 20 and the secondary sideconversion circuit 30. In addition, the allowable range may be setarbitrarily for each power supply device 101.

FIG. 2 is a block diagram of the control unit 50. The control unit 50 isa control unit having a function of controlling switching of theswitching elements such as the primary side first upper arm U1 of theprimary side conversion circuit 20 and the switching elements such asthe secondary side first upper arm U2 of the secondary side conversioncircuit 30. The control unit 50 includes a power conversion modedetermination processing unit 502, a phase difference φ determinationprocessing unit 504, an on-time δ determination processing unit 506, aprimary side switching processing unit 508, and a secondary sideswitching processing unit 510. The control unit 50 is, for example, anelectronic circuit including a micro computer having a CPU builttherein.

The power conversion mode determination processing unit 502 selects anddetermines an operation mode out of power conversion modes A to L, whichwill be described below, of the power supply circuit 10, for example, onthe basis of a predetermined external signal (for example, a signalindicating a difference between the detected value Yd and the targetvalue Yo at a certain port). The power conversion modes include mode Ain which electric power input from the first input/output port 60 a isconverted and output to the second input/output port 60 c, mode B inwhich electric power input from the first input/output port 60 a isconverted and output to the third input/output port 60 b, and mode C inwhich electric power input from the first input/output port 60 a isconverted and output to the fourth input/output port 60 d.

The power conversion modes include mode D in which electric power inputfrom the second input/output port 60 c is converted and output to thefirst input/output port 60 a, mode E in which electric power input fromthe second input/output port 60 c is converted and output to the thirdinput/output port 60 b, and mode F in which electric power input fromthe second input/output port 60 c is converted and output to the fourthinput/output port 60 d.

The power conversion modes include mode G in which electric power inputfrom the third input/output port 60 b is converted and output to thefirst input/output port 60 a, mode H in which electric power input fromthe third input/output port 60 b is converted and output to the secondinput/output port 60 c, and mode I in which electric power input fromthe third input/output port 60 b is converted and output to the fourthinput/output port 60 d.

The power conversion modes include mode J in which electric power inputfrom the fourth input/output port 60 d is converted and output to thefirst input/output port 60 a, mode K in which electric power input fromthe fourth input/output port 60 d is converted and output to the secondinput/output port 60 c, and mode L in which electric power input fromthe fourth input/output port 60 d is converted and output to the thirdinput/output port 60 b.

The phase difference φ determination processing unit 504 has a functionof setting the phase difference φ of periodic switching movement of theswitching elements between the primary side conversion circuit 20 andthe secondary side conversion circuit 30 so as to cause the power supplycircuit 10 to serve as a DC-DC converter circuit.

The on-time δ determination processing unit 506 has a function ofsetting the on-time δ of the switching elements of the primary sideconversion circuit 20 and the secondary side conversion circuit 30 so asto cause the primary side conversion circuit 20 and the secondary sideconversion circuit 30 to serve as step-up/down circuits, respectively.

The primary side switching processing unit 508 has a function ofcontrolling switching of the switching elements of the primary sidefirst upper arm U1, the primary side first lower arm /U1, the primaryside second upper arm V1, and the primary side second lower arm /V1 onthe basis of the outputs of the power conversion mode determinationprocessing unit 502, the phase difference φ determination processingunit 504, and the on-time δ determination processing unit 506.

The secondary side switching processing unit 510 has a function ofcontrolling switching of the switching elements of the secondary sidefirst upper arm U2, the secondary side first lower arm /U2, thesecondary side second upper arm V2, and the secondary side second lowerarm /V2 on the basis of the outputs of the power conversion modedetermination processing unit 502, the phase difference φ determinationprocessing unit 504, and the on-time δ determination processing unit506.

The control unit 50 is not limited to the processes illustrated in FIG.2 and can perform various processes required for controlling thetransmission power transmitted between the primary side conversioncircuit 20 and the secondary side conversion circuit 30.

<Operation of Power Supply Device 101>

The operation of the power supply device 101 will be described belowwith reference to FIGS. 1 and 2. For example, when an external signalfor requiring for selecting mode F as the power conversion mode of thepower supply circuit 10 is input, the power conversion modedetermination processing unit 502 of the control unit 50 determines thepower conversion mode of the power supply circuit 10 to be mode F. Atthis time, the voltage input to the second input/output port 60 c isstepped up by the step-up function of the primary side conversioncircuit 20, the power of the stepped-up voltage is transmitted to thethird input/output port 60 b by the function as the DC-DC convertercircuit of the power supply circuit 10, the transmitted power is steppeddown by the step-down function of the secondary side conversion circuit30, and the stepped-down voltage is output from the fourth input/outputport 60 d.

The step-up/down function of the primary side conversion circuit 20 willbe described below in detail. Paying attention to the secondinput/output port 60 c and the first input/output port 60 a, theterminal 616 of the second input/output port 60 c is connected to themidpoint 207 m of the primary side first arm circuit 207 via the primaryside first winding 202 a and the primary side first reactor 204 aconnected in series to the primary side first winding 202 a. Since bothends of the primary side first arm circuit 207 are connected to thefirst input/output port 60 a, a step-up/down circuit is disposed betweenthe terminal 616 of the second input/output port 60 c and the firstinput/output port 60 a.

The terminal 616 of the second input/output port 60 c is connected tothe midpoint 211 m of the primary side second arm circuit 211 via theprimary side second winding 202 b and the primary side second reactor204 b connected in series to the primary side second winding 202 b.Since both ends of the primary side second arm circuit 211 are connectedto the first input/output port 60 a, a step-up/down circuit is disposedin parallel between the terminal 616 of the second input/output port 60c and the first input/output port 60 a. Since the secondary sideconversion circuit 30 has substantially the same configuration as theprimary side conversion circuit 20, two step-up/down circuits areconnected in parallel between the terminal 622 of the fourthinput/output port 60 d and the third input/output port 60 b.Accordingly, the secondary side conversion circuit 30 has a step-up/downfunction similarly to the primary side conversion circuit 20.

The function as the DC-DC converter circuit of the power supply circuit10 will be described below in detail. Paying attention to the firstinput/output port 60 a and the third input/output port 60 b, the firstinput/output port 60 a is connected to the primary side full bridgecircuit 200 and the third input/output port 60 b is connected to thesecondary side full bridge circuit 300. The primary side coil 202disposed in the bridge part of the primary side full bridge circuit 200and the secondary side coil 302 disposed in the bridge part of thesecondary side full bridge circuit 300 are magnetically coupled to eachother with a coupling coefficient kT, whereby the transformer 400 servesas a center-tap transformer with a turns ratio of 1:N. Accordingly, byadjusting the phase difference φ, of the periodic switching movements ofthe switching elements of the primary side full bridge circuit 200 andthe secondary side full bridge circuit 300, the electric power input tothe first input/output port 60 a can be converted and transmitted to thethird input/output port 60 b, or the electric power input to the thirdinput/output port 60 b can be converted and transmitted to the firstinput/output port 60 a.

FIG. 3 is a diagram illustrating ON-OFF switching waveforms of the armsdisposed in the power supply circuit 10 under the control of the controlunit 50. In FIG. 3, U1 represents the ON-OFF waveform of the primaryside first upper arm U1, V1 represents the ON-OFF waveform of theprimary side second upper arm V1, U2 represents the ON-OFF waveform ofthe secondary side first upper arm U2, and V2 represents the ON-OFFwaveform of the secondary side second upper arm V2. The ON-OFF waveformsof the primary side first lower arm /U1, the primary side second lowerarm /V1, the secondary side first lower arm /U2, and the secondary sidesecond lower arm /V2 are waveforms (not illustrated) obtained byinverting the ON-OFF waveforms of the primary side first upper arm U1,the primary side second upper arm V1, the secondary side first upper armU2, and the secondary side second upper arm V2, respectively. A deadtime can be disposed between both ON and OFF waveforms of the upper andlower arms so that a penetration current does not flow at the timeturning on both of the upper and lower arms. In FIG. 3, the high levelrepresents the ON state and the low level represents the OFF state.

By changing the on-times δ of U1, V1, U2, V2, it is possible to changethe step-up/down ratio of the primary side conversion circuit 20 and thesecondary side conversion circuit 30. For example, by setting theon-times δ of the U1, V1, U2, V2 to be equal to each other, thestep-up/down ratio of the primary side conversion circuit 20 and thestep-up/down ratio of the secondary side conversion circuit 30 can beset to be equal to each other.

The on-time δ determination processing unit 506 sets the on-times δ ofU1, V1, U2, V2 to be equal to each other so that the step-up/down ratiosof the primary side conversion circuit 20 and the secondary sideconversion circuit 30 are equal to each other (on-time δ=primary sideon-time δ11=secondary side on-time δ12=time value β).

The step-up/down ratio of the primary side conversion circuit 20 isdetermined depending on the duty ratio D which is the ratio of theon-time δ to the switching period T of the switching element (arm)disposed in the primary side full bridge circuit 200. Similarly, thestep-up/down ratio of the secondary side conversion circuit 30 isdetermined depending on the duty ratio D which is the ratio of theon-time δ to the switching period T of the switching element (arm)disposed in the secondary side full bridge circuit 300. The step-up/downratio of the primary side conversion circuit 20 is a transformationratio between the first input/output port 60 a and the secondinput/output port 60 c, and the step-up/down ratio of the secondary sideconversion circuit 30 is a transformation ratio between the thirdinput/output port 60 b and the fourth input/output port 60 d.

Accordingly, for example, step-up/down ratio of the primary sideconversion circuit 20=voltage of the second input/output port 60c/voltage of the first input/output port 60 a=δ11/T=βT and step-up/downratio of the secondary side conversion circuit 30=voltage of the fourthinput/output port 60 d/voltage of the third input/output port 60b=δ12/T=βT are established. That is, the step-up/down ratio of theprimary side conversion circuit 20 and the step-up/down ratio of thesecondary side conversion circuit 30 have the same value (=β/T).

The on-time δ illustrated in FIG. 3 represents the on-time δ11 of theprimary side first upper arm U1 and the primary side second upper armV1, and represents the on-time δ12 of the secondary side first upper armU2 and the secondary side second upper arm V2. The switching period T ofthe arm disposed in the primary side full bridge circuit 200 and theswitching period T of the arm disposed in the secondary side full bridgecircuit 300 are the same time.

The phase difference between U1 and V1 is set to 180 degrees (π) and thephase difference between U2 and V2 is set to 180 degrees (#). Bychanging the phase difference φ between U1 and U2, it is possible toadjust the amount of power transmitted P between the primary sideconversion circuit 20 and the secondary side conversion circuit 30. Theelectric power can be transmitted from the primary side conversioncircuit 20 to the secondary side conversion circuit 30 when the phasedifference φ>0 is established, and the electric power can be transmittedfrom the secondary side conversion circuit 30 to the primary sideconversion circuit 20 when the phase difference φ<0 is established.

The phase difference φ is a difference in switching timing (time lag)between the power conversion circuit units of the same phase in theprimary side full bridge circuit 200 and the secondary side full bridgecircuit 300. For example, the phase difference φ is a difference inswitching timing between the primary side first arm circuit 207 and thesecondary side first arm circuit 307, and is a difference in switchingtiming between the primary side second arm circuit 211 and the secondaryside second arm circuit 311. The differences are controlled to the samestate. That is, the phase difference φ between U1 and U2 and the phasedifference φ between V1 and V2 are controlled to the same value.

Therefore, for example, when an external signal for requiring forselecting mode F as the power conversion mode of the power supplycircuit 10 is input, the power conversion mode determination processingunit 502 determines that mode F is selected. The on-time δ determinationprocessing unit 506 sets the on-time δ for defining the step-up ratiowhen the primary side conversion circuit 20 is caused to serve as astep-up circuit stepping up the voltage input to the second input/outputport 60 c and outputs the stepped-up voltage to the first input/outputport 60 a. The secondary side conversion circuit 30 serves as astep-down circuit stepping down the voltage input to the thirdinput/output port 60 b at the step-down ratio defined by the on-time δset by the on-time δ determination processing unit 506 and outputtingthe stepped-down voltage to the fourth input/output port 60 d. The phasedifference φ determination processing unit 504 sets the phase differenceφ for transmitting the electric power input to the first input/outputport 60 a to the third input/output port 60 b by a desired amount ofpower transmitted P.

The primary side switching processing unit 508 controls the switching ofthe switching elements of the primary side first upper arm U1, theprimary side first lower arm /U1, the primary side second upper arm V1,and the primary side second lower arm /V1 so that the primary sideconversion circuit 20 serves as a step-up circuit and the primary sideconversion circuit 20 serves as a part of the DC-DC converter circuit.

The secondary side switching processing unit 510 controls the switchingof the switching elements of the secondary side first upper arm U2, thesecondary side first lower arm /U2, the secondary side second upper armV2, and the secondary side second lower arm /V2 so that the secondaryside conversion circuit 30 serves as a step-down circuit and thesecondary side conversion circuit 30 serves as a part of the DC-DCconverter circuit.

As described above, the primary side conversion circuit 20 and thesecondary side conversion circuit 30 can serve as a step-up circuit or astep-down circuit and the power supply circuit 10 can serve as abidirectional DC-DC converter circuit. Accordingly, the power conversioncan be performed in all the power conversion modes A to L, that is, thepower conversion can be performed between two selected input/outputports out of four input/output ports.

The transmission power P (also referred to as amount of powertransmitted P) adjusted depending on the phase difference φ, equivalentinductance L, and the like by the control unit 50 is electric powertransmitted from one conversion circuit of the primary side conversioncircuit 20 and the secondary side conversion circuit 30 to the otherconversion circuit via the transformer 400, and is expressed byExpression (1), P=(N×Va×Vb)/(π×ω×L)×F(D, φ)).

Here, N represents the turns ratio of the transformer 400, Va representsthe input/output voltage of the first input/output port 60 a (thevoltage between the primary side positive electrode bus line 298 and theprimary side negative electrode bus line 299 of the primary sideconversion circuit 20), and Vb represents the input/output voltage ofthe third input/output port 60 b (the voltage between the secondary sidepositive electrode bus line 398 and the secondary side negativeelectrode bus line 399 of the secondary side conversion circuit 30). πrepresents the circular constant and ω (=2π×f=2π/T) represents theangular frequency of the switching of the primary side conversioncircuit 20 and the secondary side conversion circuit 30. f representsthe switching frequency of the primary side conversion circuit 20 andthe secondary side conversion circuit 30, T represents the switchingperiod of the primary side conversion circuit 20 and the secondary sideconversion circuit 30, and L represents the equivalent inductanceassociated with the transmission of electric power of the magneticcoupling reactors 204, 304 and the transformer 400. F(D, φ)) is afunction having the duty ratio D and the phase difference φ asparameters and is a parameter monotonously increasing with the increasein the phase difference φ without depending on the duty ratio D. Theduty ratio D and the phase difference φ are control parameters designedto vary within a range of predetermined upper and lower limits.

The equivalent inductance L can be defined in an equivalent circuit ofthe transformer 400 connected to the primary side magnetic couplingreactor 204 and/or the secondary side magnetic coupling reactor 304. Theequivalent inductance L is combined inductance obtained by combiningleakage inductance of the primary side magnetic coupling reactor 204and/or the leakage inductance of the secondary side magnetic couplingreactor 304 and the leakage inductance of the transformer 400 in thesimple equivalent circuit.

For example, the equivalent inductance L (secondary side converted valueL_(EQ2)) measured from the secondary side conversion circuit 30 can beexpressed by Expression (2),L_(EQ2)=2L₁(1−k₁)N²+2L₂(1−k₂)+L_(T2)(1−k_(T) ²).

L₁ represents the self inductance of the primary side magnetic couplingreactor 204, k₁ represents the coupling coefficient of the primary sidemagnetic coupling reactor 204, N represents the turns ratio of thetransformer 400, L₂ represents the self inductance of the secondary sidemagnetic coupling reactor 304, k₂ represents the coupling coefficient ofthe secondary side magnetic coupling reactor 304, L_(T2) represents theexciting inductance on the secondary side of the transformer 400, andk_(T) represents the coupling coefficient of the transformer 400. Whenthe second input/output port 60 c or the fourth input/output port 60 dis not used, the leakage inductance appearing in the first term or thesecond term in Expression (2) may be absent.

The control unit 50 adjusts the transmission power P by changing thephase difference φ so that the port voltage Vp of at least one port ofthe primary-ports and the secondary-ports converges on a target portvoltage Vo. Accordingly, even when the current consumption of a loadconnected to the port increases, the control unit 50 can prevent theport voltage Vp from departing from the target port voltage Vo bychanging the phase difference φ to adjust the transmission power P.

For example, the control unit 50 adjusts the transmission power P bychanging the phase difference φ so that the port voltage Vp of the otherport as the transmission destination of the transmission power P out ofthe primary side ports and the secondary side ports converge on thetarget port voltage Vo. Accordingly, even when the current consumptionof a load connected to the port as the transmission destination of thetransmission power P increases, the control unit 50 can prevent the portvoltage Vp from departing from the target port voltage Vo by increasingthe phase difference φ to adjust the transmission power P.

FIG. 4 is a block diagram illustrating a configuration example of thecontrol unit 50 for calculating a PID calculated value. The control unit50 includes a PID control unit 51 and the like. The PID calculated valueis, for example, a command value φo of the phase difference φ and acommand value Do of the duty ratio D.

The PID control unit 51 includes a phase difference command valuegenerator that generates the command value φo of the phase difference φfor causing the port voltage of at least one port out of the primaryside ports and the secondary side ports to converge on the targetvoltage by PID control for each switching period T. For example, thephase difference command value generator of the PID control unit 51generates the command value φo for causing the difference to converge onzero for each switching period T by performing the PID control on thebasis of the difference between the target voltage of the port voltageVa and the detected voltage of the port voltage Va acquired by thesensor unit 70.

The control unit 50 adjusts the transmission power P determined byExpression (1) by performing the switching control of the primary sideconversion circuit 20 and the secondary side conversion circuit 30 onthe basis of the command value φo generated by the PID control unit 51so that the port voltage converges on the target voltage.

The PID control unit 51 includes a duty ratio value generator thatgenerates the command value Do of the duty ratio D for causing the portvoltage of at least one port out of the primary side ports and thesecondary side ports to converge on the target voltage by the PIDcontrol for each switching period T. For example, the duty ratio commandvalue generator of the PID control unit 51 generates the command valueDo for causing the difference to converge on zero for each switchingperiod T by performing the PID control on the basis of the differencebetween the target voltage of the port voltage Vc and the detectedvoltage of the port voltage Vc acquired by the sensor unit 70.

The PID control unit 51 may include an on-time command value generatorgenerating a command value δo of the on-time δ instead of the commandvalue Do of the duty ratio D.

The PID control unit 51 adjusts the command value φo of the phasedifference φ on the basis of an integral gain I1, a differential gainD1, and a proportional gain P1, and adjusts the command value Do of theduty ratio D on the basis of an integral gain I2, a differential gainD2, and a proportional gain P2.

Note that, a relationship of port voltage Va×duty ratio D=port voltageVc is established among the port voltage Va, the port voltage Vc, andthe duty ratio D. Accordingly, when it is wanted to step down theconstant port voltage Va (for example, 10 V) to increase the portvoltage Vc (for example, from 1 V to 5 V), the duty ratio D can beincreased (for example, from 10% to 50%). On the contrary, when it iswanted to step up the constant port voltage Vc (for example, 5 V) toincrease the port voltage Va (for example, from 10 V to 50 V), the dutyratio D can be decreased (for example, from 50% to 10%). That is, thePID control unit 51 inverts the control direction of the duty ratio D(the direction in which the duty ratio D increases or decreases) in thestep-up operation and the step-down operation by switching the controltarget (the first input/output port 60 a or the second input/output port60 c).

<Relationship Between Transmission Power and Voltage Ratio M>

Herein, the relationship between the transmission power and the voltageratio M will be described with reference to FIG. 5A, FIG. 5B and FIG.6A, FIG. 6B.

FIG. 5A and FIG. 6A show a case where the voltage ratio M is normal,while FIG. 5B and FIG. 6B show a case where the voltage ratio Mdeviates. The horizontal axis represents time T, and the vertical axisrepresents a current I. Area corresponds to the transmission power P.

When the voltage ratio M is normal, the upper side of the graph ishorizontal (see FIG. 5A, FIG. 6A), and when the voltage ratio Mdeviates, the upper side of the graph is oblique (see FIG. 5B, FIG. 6B).

It is known from FIG. 5A and FIG. 5B that the peak values I1 of thecurrents I are equal, and the transmission power P when the voltageratio M is normal is greater than the transmission power P when thevoltage ratio M deviates (the area of the graph of FIG. 5A>the area ofthe graph of FIG. 5B). In other words, since the voltage ratio Mdeviates, the transmission power P may decrease.

On the other hand, it is known from FIG. 6A and FIG. 6B, thetransmission power P is equal (the areas are equal), and the peak valueI1 of the current I when the voltage ratio M is normal is less than thepeak value I2 of the current I when the voltage ratio M deviates (thepeak value I1 in FIG. 6<the peak value I2 in FIG. 6B). In other words,since the voltage ratio M deviates, the peak value of the current Iincreases even if the transmission power P is equal.

It is known from FIG. 5A, FIG. 5B and FIG. 6A, FIG. 6B, as compared tothe case that the voltage ratio M deviates (the voltage ratio M deviatesfrom the reference value by the specified value or more), in the casethat the voltage ratio M is normal (the voltage ratio M does not deviatefrom the reference value by the specified value or more), thetransmission power is prone to decrease and the peak current value isprone to increase.

When the transmission power decreases, in order to compensate for thedecrease in the transmission power P, it is necessary to perform anexcess amount of element design, which increases the cost of the powersupply device 101. Further, when the peak current value increases, sincethe current that flows into the transformer 400, the primary sidemagnetic coupling reactor 204, the secondary side magnetic couplingreactor 304, the respective arms, and the like increases, it isnecessary to enhance ratings of elements and expand sizes of theelements, which increases the cost of the power supply device 101.Therefore, in order to perform a high precision power transmission inthe power supply device 101, it is important to maintain the portvoltage Vb to be a value within the specified range so that the voltageratio M does not deviate from the reference value by the specified valueor more.

In accordance with the power supply device 101 according to the presentembodiment, the control unit 50 causes the step-up/down DC-DC converter80 that is connected to the third input/output port 60 b to operate at asuitable timing (if necessary). Thus, since it is possible to keep thevoltage ratio substantially constant even if the voltage on the side ofthe secondary side high voltage system power supply variessignificantly, it is possible to suppress the decrease in thetransmission power and the increase in the peak current value, andreduce the cost of the power supply device 101.

Further, the components (for example, a coil component, and the like)used in the step-up/down DC-DC converter 80 that is connected to thethird input/output port 60 b and the components used in the primary sideconversion circuit 20 and the secondary side conversion circuit 30 (forexample, a filter L, and the like) can be shared. Thus, it is possibleto reduce the labor and cost of adding new components. In addition, itis possible to reduce the current relatively by inserting thestep-up/down DC-DC converter to the side of the high voltage systempower supply (on the side of the third input/output port 60 b). Thereby,since it is possible to suppress the element rating of the step-up/downDC-DC converter itself, the increase in cost can be limited to theminimum degree.

<Operation Flow of Power Supply Device 101>

FIG. 7 is a flowchart illustrating an example of the power conversionmethod. The power conversion method illustrated in FIG. 7 is performedby the control unit 50.

In step S310, the control unit 50 monitors the voltage ratio M (theratio of the port voltage Va and the port voltage Vb).

For example, a case that the port voltage Va:the port voltageVb=50V:300V=1:6 (the reference value of the voltage ratio M=6), and thespecified value is ±2 (a deviation rate is ±33%) is considered.

When the port voltage Vb is changed from 300V to 240V, since the portvoltage Va:the port voltage Vb=50V: 240V=1:4.8, the voltage ratio M is4.8. Since the voltage ratio M is changed from the reference value (=6)by 1.2 (=−4.8), it does not deviate by the specified value (=±2) or more(the deviation rate is (1.2/6)×100=20%). Therefore, there is no need forthe control unit 50 to cause the step-up/down DC-DC converter 80 tooperate.

When the port voltage Vb is changed from 300V to 420V, since the portvoltage Va:the port voltage Vb=50V:420V=1:8.4, the voltage ratio M is8.4. Since the voltage ratio M is changed from the reference value (=6)by −2.4 (=6−8.4), it deviates by the specified value (=±2) or more (thedeviation rate is (2.4/6)×100=40%). Therefore, there is a need for thecontrol unit 50 to cause the step-up/down DC-DC converter 80 to operate.

In this way, in order to determine whether it is necessary to cause thestep-up/down DC-DC converter 80 to operate, the control unit 50 monitorsthe voltage ratio M based on the detected voltage of the port voltage Vaand the detected voltage of the port voltage Vb.

In step S320, the control unit 50 determines whether the voltage ratio Mdeviates from the reference value by the specified value or more(whether the port voltage Vb varies beyond the specified range). Whenthe voltage ratio M deviates from the reference value by the specifiedvalue or more (YES), the control unit 50 performs the processing of stepS330. When the voltage ratio M does not deviates from the referencevalue by the specified value or more (NO), the control unit 50 performsthe processing of step S340.

According to the determination in step S320, the control unit 50 candetermine whether it is necessary to cause the step-up/down DC-DCconverter 80 to operate.

In step S330, the control unit 50 causes the step-up/down DC-DCconverter to operate, and then returns to step S320 again. The controlunit 50 repeats step S320 and step S330 until the voltage ratio M doesnot deviate from the reference value by the specified value or more.

In step S340, the control unit 50 causes the step-up/down DC-DCconverter to not operate, and returns to step S310 again.

As described above, the control unit 50 monitors the voltage ratio M bythe control in step S310, and determines whether the voltage ratio Mdeviates from the reference value by the specified value or more by thecontrol in step S320. Then, when the voltage ratio M deviates from thereference value by the specified value or more, the control unit 50causes the step-up/down DC-DC converter to operate by the control instep S330, and when the voltage ratio M does not deviate from thereference value by the specified value or more, the control unit 50causes the step-up/down DC-DC converter to not operate by the control instep S340. That is, when the voltage of the secondary side port variesgreatly, the control unit 50 causes the step-up/down DC-DC converter tooperate, to keep the voltage ratio of the voltage of the primary sideport and the voltage of the secondary side port substantially constant.Thus, it is possible to suppress the decrease in the transmission powerin the power supply device 101.

While the power conversion device and the power conversion method havebeen described above with reference to the embodiment, the invention isnot limited to the aforementioned embodiment. Various modifications andimprovements such as combination or replacement of a part or all ofother embodiments can be made without departing from the scope of theinvention.

For example, in the aforementioned embodiment, a MOSFET as asemiconductor element that is turned on or off has been used as anexample of the switching element. However, the switching element may bea voltage-controller power element using an insulating gate such as anIGBT or a MOSFET or may be a bipolar transistor.

A power supply may be connected to the first input/output port 60 a or apower supply may be connected to the fourth input/output port 60 d.

The secondary side may be defined as the primary side and the primaryside may be defined as the secondary side.

The invention can be applied to a power conversion device that includesthree or more input/output ports and that can convert electric powerbetween two input/output ports out of the three or more input/outputports. For example, the invention can be applied to a power supplydevice having a configuration in which any one input/output port out offour input/output ports illustrated in FIG. 1 is removed.

What is claimed is:
 1. A power conversion method of a power conversiondevice including a primary side port disposed in a primary side circuitand a secondary side port disposed in a secondary side circuitmagnetically coupled to the primary side circuit with a transformer, thepower conversion device adjusting transmission power transmitted betweenthe primary side circuit and the secondary side circuit by changing aphase difference between switching of the primary side circuit andswitching of the secondary side circuit, and changing a voltage of thesecondary side port by a DC-DC converter that is connected to thesecondary side port, the power conversion method comprising: monitoringa voltage ratio of a voltage of the primary side port and the voltage ofthe secondary side port; determining whether the voltage ratio deviatesfrom a reference value by a specified value or more; and causing theDC-DC converter to operate when the voltage ratio deviates from thereference value by the specified value or more.
 2. The power conversionmethod according to claim 1, wherein the voltage of the secondary sideport is maintained to be a value within a specified range.
 3. The powerconversion method according to claim 1, wherein the voltage ratio isdependent on the voltage of the secondary side port.
 4. The powerconversion method according to claim 1, wherein elements included in theDC-DC converter and components included in the primary side circuit andthe secondary side circuit are shared.
 5. A power conversion devicecomprising: a primary side circuit including a primary side port; asecondary side circuit including a secondary side port and beingmagnetically coupled to the primary side circuit with a transformer; acontrol unit controlling transmission power transmitted between theprimary side circuit and the secondary side circuit by changing a phasedifference between switching of the primary side circuit and switchingof the secondary side circuit; and a DC-DC converter that is connectedto the secondary side port and changes a voltage of the secondary sideport, wherein the control unit is configured to monitor a voltage ratioof a voltage of the primary side port and the voltage of the secondaryside port; determine whether the voltage ratio deviates from a referencevalue by a specified value or more; and cause the DC-DC converter tooperate when the voltage ratio deviates from the reference value by thespecified value or more.
 6. The power conversion device according toclaim 5, wherein the voltage of the secondary side port is maintained tobe a value within a specified range.
 7. The power conversion deviceaccording to claim 5, wherein the voltage ratio is dependent on thevoltage of the secondary side port.
 8. The power conversion deviceaccording to claim 5, wherein elements included in the DC-DC converterand components included in the primary side circuit and the secondaryside circuit are shared.